1. Field of the Invention
This invention relates to a stress/torque sensor based on the principle of magnetostriction, and more particularly, to a simplified magnetostrictive stress/torque sensor which employs a minimal number of elements and hence is simpler and more economical than state of the art sensors.
2. Description of Prior Art
Engineers and scientists have sought a simple, reliable, accurate means for measuring torque in rotating shafts for well over a century. Applications for such a torque measuring apparatus include diagnosis, prognosis, and load level monitoring of a vast number of different types of rotary drive mechanisms such as automotive, ship, and plane engines; motors and generators of all types; oil drilling rigs; rotating machining tools; all electric power steering; robotics; and much more.
Further, measurement of mechanical power produced by an engine (or used by a generator) cannot be made without knowing both torque and rotational speed of the shaft. Hence there has heretofore been no ready means to determine on-line power and efficiency of rotary drive devices simply, accurately, and reliably. This has proven to be problematic in many areas of modern technology, but it has been particularly troublesome in attempts to develop modern automotive engine control systems which would improve fuel efficiency and optimize engine performance.
There are presently only four distinct methods for measuring torque directly in a rotating shaft. They are:
1. Twist angle of shaft measurement PA0 2. Strain gauge sensor PA0 3. Reaction force measurement PA0 4. Magnetostrictive sensors PA0 1) Non-contact: no slip rings PA0 2) Not restricted to low speeds PA0 3) Measures torque of engine directly PA0 4) High sensitivity PA0 5) Economical PA0 6) Structural advantages, i.e., no strain gauge attachment problems, no large apparata PA0 7) Only one location anywhere along shaft axis: little engine rework PA0 8) Durable and reliable: no moving parts to cause mechanical failure, resistant to high pressure and temperature of engine environment PA0 9) Readily miniaturized: can be made unobtrusive
The twist angle method involves measurement of the angle of twist of a shaft and correlates this, using the material and dimensional characteristics of the shaft, to torque. It entails a complicated and cumbersome mechanism with low sensitivity, calibration difficulties, and the necessity of using two different locations along the shaft. It invariably entails extensive engine modification, a costly endeavor.
The strain gauge approach requires bonding of strain guages to the shaft surface and relating strain measurement to torque. It is limited to low speed, is not amenable to mass production, lacks durability, and needs some means such as slip rings and brushes to bring the signal off of the shaft.
Reaction force measurement utilizes Newton's second law for rotational motion to relate force and motion of the engine mounts to shaft torque. The method must employ a large structure, has low sensitivity, is not feasible for production runs, and measures driveline, not engine, torque.
Magnetostrictive torque sensors take advantage of the magnetostrictive property of many ferromagnetic materials whereby tension stress typically increases (and compressive stress typically decreases) a given magnetic induction field (i.e., the "B" field) carried by the material. A coil of wire of arbitrary number of turns wrapped around an iron core is placed close to the shaft and an electric current passing through the wire causes a magnetic field to be induced in the rotating shaft. In magnetostrictive sensor designs such as those described in U.S. Pat. Nos. 4,590,807 and 4,627,298 and SAE paper #890482, a second coil of arbitrary number of turns is wrapped around the same iron core and used to measure the change in the induction (the B field) which results from the increased surface stress caused by the applied torque. Other magnetostrictive designs employ additional cores and/or coils in configurations differing from the aforementioned. These other designs are inherently more complex in nature, require additional elements, and for these reasons are not further discussed herein.
The magnetostrictive method described above has several advantages over the other three torque measuring methods, including